Method of making mosaic electrodes



METHOD OF MAKING MOSAIC ELECTRODES Filed March 26, 1938 amr.

INVEN TOR. WILLRD HICKOK BY www' A TTORNEY.

Patented UNITED STAT METHOD F MAUNG MOSAIC ELECTRODES DelawareApplication March 26, 1938, Serial No. 198,209

ZCIaims.

My invention relates to improvements in cathode ray television tubesandparticularly to improvedA methods of manufacturing television tubeswhich have photosensitive mosaic electrodes.

It has been found that in cathode ray tubes with a conventional mosaicelectrode having on the front surface of a mica sheet discrete mutuallyisolated elements or particles of silver photosensitized by exposure toan alkali metal vapor such as caesium vapor the best results areobtained when the tube is treated during exhaust to render the surfaceconductivity and resultant leakage between particles very low. It hasbeen found that notwithstanding such treatment during exhaust, a certainamount of residual electrical leakage takes place between thephotosensitive particles thereby limiting the resolving power of thedevice, and that further treatment to reduce this electrical leakagemakes the photosensitivity of the silver particles considerably lessthan the maximum obtainable.

The principal object of my invention is to provide an improved cathoderay tube having a mosaic type target electrode of greaterphotosensitivity and with less electrical leakage between the particlesthan a conventional type electrode. It is another object to provide animproved and highly sensitized electrode having greater photosensitivityand resolving power than has heretofore been obtainable. It is a furtherobject of my invention to provide a method whereby the photosensitivityand resolving power of a mosaic electrode may be controlled duringmanufacture.

In accordance with my invention the target electrode having amultiplicity of mutually separated metallic particles is photosensitizedin the usual way, such as by exposing the electrode supporting theparticles to the vapor of an alkali metal such a-s caesium andsubsequently oxidizing or wholly eliminating the metallic caesium whichis deposited between the photosensitive particles. This step ofeliminating the metal between the particles is preferably performedwhile the tube is in normal operation. Further in accordance with myinvention the electrode from which the free caesium has been removedbetween the particles is subsequently baked to increase thephotosensitivity of the particles.

Other objects, features and advantages of my invention will appear fromthe following description taken in connection with the accompanyingdrawing in which:

Figure 1 is a diagrammatic view illustrating (Cl. Z50-27.5)

one form of a television device made in accordance with my invention,

Figure 2 is an enlarged fragmentary sectional view of the photosensitivemosaic electrode during the process of formation and,

Figure 3 is an enlarged fragmentary sectional view made in accordancewith my improved method.

A tube made in accordance with my invention is shown in Figure 1 andcomprises a highly evacuated glass envelope or bulb I with a tubular armor neck section enclosing a conventional type electron gun and acylindrical section enclosing a flat target or mosaic electrode 2 sopositioned that its front surface may be scanned by a beam of electronsfrom the electron gun and also may have projected upon it the opticalimage to be transmitted. Since the image is produced from an objectsituated outside the tube, that portion or window 3 of the envelopeopposite the electrode 2 is made optically uniform so that the image tobe transmitted may be projected upon the electrode with a minimum ofdistortion by the lens system i.

The electron gun assembly is of the conventional type, and comprises acathode E from which an electron stream may be drawn, a controlelectrode 6 connectedto the usual biasing battery, and a rst anode 'Imaintained positive with respect to the cathode by a battery 8. Theelectron stream leaving the first anode 1 is accelerated andconcentrated into an electron scanning beam focused on the front surfaceof the mosaic electrode 2 by a second anode 9, which is preferably aconductive coating on the surface of the bulb I extending into the neckof the bulb but removed from that portion through which is projected theoptical image to be transmitted. Conventional deflection means, such asdeection coils I0 and II may be used to sweep the beam in a horizontaland Vertical plane, respectively, to scan the target. It is obvious thatconventional electrostatic deiiection plates may be substituted for oneor both of the deectlon coils if desired. The electrode 2 is connectedthrough the impedance I2 to ground andto the collector electrode orsecond anode 9, and in operation the current ow in this circuit producesa voltage drop across the impedance I2 which may be impressed on theinput of a translating device I3, further amplified and applied to atransmitting network in a manner well known to the art.

The mosaic electrode as best shown in Figure 2 comprises an insulatingfoundation sheet or lasl a signal electrode from which the picturesignals may be obtained. The other side of the sheet of mical has on itssurface a plurality of mutually isolated metallic particles Il.

In accordance with my invention the metal particles are photosensitisedduring the evacuating process of the tube in which the electrode is usedby oxidizing the metal particles and sub- Jecting the electrode andoxidized particles to the vapor of an alkali metal such. as caesiumwhich deposits not: only on the metal particles Il but also upon thebase Il between the particles which metal between the particles isconverted into an electrically non-conducting medium, by subjecting theelectrode assembly to a controlled quantity of dry oxygen which reactswith the free alkali metal on the base between the particles to form anelectrically nonconducting material.

In making the mosaic electrode 2, I select a sheet of mica or other goodinsulating material having a thickness of* approximately .002" and anarea sunlcient to satisfy the operating requirements of the deviceinwhich the electrode is to be used. One surface of the mica sheet isdusted with a finely divided metal compound, such as silver oxide, andthe dusted sheet is rapidly heated in an oven at 800 C. until the silveroxide is reduced to silver of which individual and minute portions aredrawn up by surface tension to form a multiplicity oi' microscopicparticles I8 spaced from each other, the number and average size of theparticles in a imit area being sunlcient to satisfy the operatingrequirements in the way of detail of picture reproduction. The mosaicsurface may be formed, however, in a variety of ways, thus for example.instead of using the metal compound which is applied to the mica baseand subsequently reduced and` drawn up by surface tension intoindividual particles, a thin continuous nlm of silver may be depositedon the base, which nlm is then broken up into microscopic particles bysuitable heat treatment.

The mica sheet or base Il carrying the silver particles Il is thenmounted in the tube, as shown in Figure l, the tube evacuated and thesilver particles oxidized by admitting oxygen to the envelope to form anoxide coating I'I on each of the silver particles. The excess oxygen isremoved from the bulb and the oxidized particles are photosensitized byexposing them to vapor of an alkali metal such as caesium. Aconventional way of performing the sensitizing step is to provide asource oi' caesium, such as a reduoible compound of caesium Il intheannulus In, which annulus following the photosensitizing step issealed oil and removed from the bulb I at the point 2l.

During the exposure of the particles to caesium vapor some of thecaesium condenses on the surface of the mica base as a metallic nlm 2|which covers both the oxidized particles Il and the base Il between theparticles thereby forming an electrical leakage path between theparticles and consequently reducing the electrical resistance fromparticle to particle. A portion of this caesium nlm 2| which is betweenthe par ticles may be removed during the evacuating process. However, ifsuilicient caesium is deposited on the particles to provide optimumphoj:

tosensitisstion, the residual caeshnn between the insulatignubetw l, thedepositionofcaesimthetubeisbaked andsimultaneously pumped until maxlminnphotosmsitivity is obtained and during this baking and pumping some ofthe free caesium previously condensed-as the nlm-2| on the mica basebetween the particles is removed. The baking is discontinued when thephotosensitivlty has increasedtoamaximumandhasdecreasedvery slightly atwhich Point the leakage between the particles due to the presence ofcaesium between the particles may be still too excessive to satisfy theoperating requirements of the device.

In accordance with my invention, the electrodes of the tube areconnected, as shown in Figure l, an optical image of an object projectedon the mosaic electrode 2, and the mosaic electrode scanned with thecathode rarbeam. The

picture signals derived from the signal electrode Il are amplined by aseries of amplifiers, such as the ampliner Il, and the optical imageprojected on the mosaic electrode -2 is recreated on a viewing tube (notshown). The resolution of the recreated image, which may be denned asthe ability to distinguish two lines of an image which are closetogether, is noted. Oxygen is then liberated within the tube and in thepresence of 'the mosaic electrode. the liberation of oxygen beingcontinued until the resolving power of the mosaic electrode whichdetermines the resolution of the recreated image has passed through amaximum. This indicates that the insulation between the particles hasbecome greater. the metal nlm 2| being converted into an electricallynon-conducting material such as the oxide of the metal. .At the sametime the operating photosensitivity of the device likewise increases.Care should be taken that just sunicient oxygen is liberated to bringthe resolution and the photosensitivity to a maximum' as it has beenfound that following the point at which maximum photosensitivity isobtained, the photosensitivity rapidly decreases with the admission ofadditional oxygen. Care should likewise be taken that the oxygen is dryas it has been found that if any moisture is present thephotosensitization will be decreased I prefer to provide a source ofoxygen within the tube capable of liberating oxygen in the amountrequired. For this purpose I provide a source of oxygen such as thenickel nlament 22 which contains absorbed oxygen and` which is locatedon the inside wall of the bulb I to the rear of the mosaic electrode. Ihave found that a nickel'ribbon nlament having a length of 1.5",

, which combines with the caesium on .the mica.

base between the particles. This caesium! is converted to caesium oxidewhich is a relatively poor electrical conductor. 'I'he caesium nlm 2Ishown in Figure 2 is thus converted into a nlm of non-conductingmaterial such as the caesium oxide nlm 24 shown in Figure 3.

Following the liberation of oxygen to the proper amount, as indicated bythe operation of the device, the photosensitivity of the mosaicelectrode is higher than that which is observed prior to the liberationof the oxygen, but may be subsequently increased to a still higher valueby baking the tube at a temperature of 200 C. for approximately 7 to 10minutes or until maximum photosensitivity is obtained.

Television transmitting tubes when made in accordance with my inventionhave shown higher sensitivities to light and higher resolution thantubes not made in accordance with my new and improved method. Tubes madein the manner disclosed are more uniform with respect tophotosensitivity and resolving power and it has been found that tubeslow in photosensitivity and resolving power may be improved considerablyby following the teachings of my invention.

While I do not wish to be limited by any particular theory to explainthe improved sensitivity and increased resolution of a television devicemade in accordance with my invention, it seems probable that the oxygenwhich is liberated within the tube first reacts with the metalliccaesium between the photosensitized metal particles, oxidizing thiscaesium to caesium oxide which is a relatively poor electricalconductor. It is also believed that the oxygen reacts with part of thevfree caesium present on the oxidized silver particles, therebyincreasing the photosensitivity. During the subsequent baking operationit is believed that free caesium which has been adsorbed by the oxidizedsilver particles is driven to the surface of the particles, but thatthis caesium does not migrate to the intervening areas of the basemember because the resolution following the baking step is as high as orhigher than the resolution prior to baking.

d While I have indicated the preferred embodilimited to the exact formsillustrated or the use indicated, but that many variations may be madein the particular structure used and the purpose for which it isemployed without departing from the scope of my invention as set forthin the appended claims.

I claim:

l. The method of photosensitizing a mosaic electrode having a pluralityof mutually separated metallic particles on an insulating base supportedwithin a cathode ray tube wherein a cathode ray beam for scanning isdeveloped, which comprises depositing a lm of alkali metal on saidmetallic particles and said base, thermally removing a portion of thealkali metal from areas of the base intermediate the particles,projecting an optical image on said mosaicelectrode, tracing thedeveloped cathode ray beam across the said particles to produce signalsrepresentative of the surface condition of said electrode, liberatingoxygen within said envelope and limiting the amount of liberated oxygento the point of maximum photosensitivity and resolving power of themosaic electrode in accordance with observations of the signalsresulting from said scanning.

2. The method of photosensitizing a mosaic electrode having a pluralityof mutually separated metallic particles on an insulating base supportedwithin a cathode ray tube wherein a cathode ray beam for scanning isdeveloped, which comprises depositing a lm of caesium on lsaid metallicparticles and said base, thermally removing a portion of the caesiumfrom areas of the base intermediate lthe particles, projecting anoptical image on said mosaic electrode, tracing the developed cathoderay beam across the said particles to produce signals representative ofthe surface condition of said electrode, liberating oxygen within saidenvelope and limiting the amount of liberated oxygen to the point ofmaximum photosensitivity and resolving power of the mosaic electrode inaccordance with observations of the signals resulting from saidscanning, and subsequentlyheating the mosaic electrode to a temperatureof approximately 200 C. for a predetermined period of time tophotosensitize said metallic particles.

WIILARD HICKOK.

